In industrial induction heat processing of a strip material or slab material, the fixed interior heating cross sectional opening of a rectangular sheet inductor or a multi-turn inductor through which the material passes is sized to achieve a required process level of induced material heating. The strip or slab material may have an abnormal region, for example, where the leading (head) or trailing (tail) of the strip or slab is either not flat (that is, curved upward or downward) or the cross sectional thickness of the material is greater than the interior heating cross sectional opening of the inductor. For example as shown in FIG. 1(a) the abnormal region is the head end 90a of the material 90 (with normally constant thickness z3) curved upward to prevent passage of the material into the interior of inductor 11 with fixed cross sectional interior height z2 and positioned relative to the height of the curved head end of the material so that the curved head end would collide with the inductor and not pass within the inductor. An example of an abnormal region is shown in FIG. 1(b) and FIG. 1(c) where the abnormal region is the head end 90b of the material 90 having a maximum head end 90b thickness of z1 while the interior height z2 (in the Z-direction) of the required interior heating cross sectional opening z2 of rectangular sheet inductor 11 is less than z1 but greater than z3 which is the normally constant thickness of the strip or slab region to be induction heat treatment. In both examples the interior cross sectional opening of the inductor would be fixed for efficient induction heating of the material passing through the inductor. Therefore there is the need for a rectangular sheet inductor or a multi-turn inductor that can have a temporarily extendible cross sectional interior opening to allow an abnormal region of the material to pass through the inductor with return to a smaller interior cross sectional opening for efficient induction heating of the material outside of the abnormal region particularly in industrial processes where multiple strips or slabs are sequentially passing through the inductor.
In addition to the requirement to allow an abnormal region of the material to pass through the inductor, in some industrial induction heat processing of a strip or slab material such as a multiphase steel composition (for example dual phase steels, transformation induced plasticity (TRIP) steels, ferrite-bainite (FB) steels and complex phase (CP) steels) where the impedance of the (load) material varies as it passes through the rectangular sheet inductor or a multi-turn inductor, there is a need for load impedance match with the output of the induction power source supplying power to the inductor. Therefore there is the need for a rectangular sheet inductor or a multi-turn inductor that has an extendible cross sectional opening of the rectangular sheet inductor or a multi-turn inductor to allow an abnormal region of the material to pass through the inductor and to return to a smaller interior heating cross sectional opening and, either alternatively, or in combination therewith, to match load impedance of a material with varied electromagnetic properties by varying the cross sectional opening of the rectangular sheet inductor or a multi-turn inductor as the material passes through the inductor particularly when multiple strips or slabs are sequentially passing through the inductor and the change in the material's electromagnetic properties are randomly variable as the material passes through the inductor.
Where there is an industrial induction heating process that requires both increasing the interior cross sectional opening of the inductor for passage of an abnormal region of the material, and changing the interior cross sectional opening for variable load impedance matching there is the need for a rectangular sheet inductor or a multi-turn inductor to have separate features for performing each of these two functions.